The ultrasound source in question is based upon opto-acoustic generation of ultrasound waves by the thermo-elastic effect, in which the acoustic wave results from the interaction of a medium with a laser beam. The laser beam impinges upon the medium, and the reaction of the latter causes generation of a pressure wave in the surrounding environment. There exist various possibilities for generating ultrasound waves using laser pulses. In the present situation, the acoustic wave is generated by the thermo-elastic effect: the material impinged upon by the laser pulse heats up abruptly, and the consequent thermal expansion gives rise to the ultrasound wave.
Thermo-elastic generation of ultrasound waves is interesting because it does not entail damage to the material impinged upon by the radiation and because it does not require high-power laser sources. However, it has never found a consolidated practical or commercial application, on account of the extremely low conversion efficiency of the devices so far developed.
The Italian Patent No. 1 286 836 filed on Sep. 20, 1996 describes an opto-acoustic transducer for generating ultrasound waves, which comprises an optical fiber for conveying a laser beam and an element associated to said fiber and arranged in such a way that the laser beam impinges upon said element, which absorbs only partially the energy of said beam, converting it into thermal energy. The thermal shock induced by said conversion brings about the formation of ultrasound waves by the thermo-acoustic effect. The element consists of an opto-acoustic conversion layer applied on a portion of the optical fiber, and this conversion layer is generally metallic and consequently reflects a high percentage of the energy which reaches it, thus markedly reducing efficiency in transduction into ultrasound waves. The use of an antireflecting layer, such as a layer of dielectric material, has not yielded satisfactory results. The thin metallic layer frequently melts when the energy that impinges upon it exceeds certain limits.